The Global Positioning Satellite (hereinafter "GPS") system includes a constellation of orbiting satellites which transmit coded information enabling a receiver-equipped observer to determine its own position and velocity. In the GPS system, each satellite utilizes the same fundamental carrier frequency to modulate its code encoded information bearing transmitted signal. The carrier frequency is first coded by a pseudo-random noise code uniquely identifying the individual satellite. A pseudo-random noise code is normally a repeating code which has random noise-like properties. In particular, the autocorrelation of a pseudo-random noise code approaches zero at all times except at zero delay.
In practice, the GPS system utilizes Gold codes, a form of pseudo-random coding, to phase modulate the carrier and spread the spectrum of the modulated information to combat interference and perform a variety of GPS-related functions. In the GPS system each satellite transmits several Gold encoded signals. While commercial users of the GPS system generally use what are known as C/A codes, the GPS satellites also transmit a P code encoded carrier which is intended primarily for military use. The C/A Gold codes for each satellite are published, whereas the P codes which are also Gold code modulated, are restricted due to their military nature.
The C/A code is encoded on a carrier F.sub.0 /10=1.023 MHz.
As mentioned above, each satellite utilizes a different Gold code as the spread spectrum C/A code. The spread spectrum carrier is modulated with an information signal containing data transmitted at 50 hertz. The differential phase shift keyed (DPSK) data is added to the spread spectrum carrier.
Accessing a GPS data stream requires a reversal of the above mentioned encoding process. Several types of information are thus derivable by decoding the spread spectrum positional signals developed by each GPS satellite. Such GPS receivers are of course known. Such receivers conventionally utilize a code tracking loop to de-spread the spread spectrum Gold code to recover the information contained within the code. The code tracking loop further phase locks an internally generated pseudo-random noise code to the incoming code to both remove the code and to establish the propagation delay between the satellite and receiver. This propagation delay defines the pseudo-range between the satellite and receiver. This propagation delay is not determinative of actual distance or range because the repeat time of the C/A code is substantially less than the distance being measured. The C/A code repeats itself approximately once per millisecond. The transmitted signal will only propagate about 293 meters during this time and thus, the pseudo-range is the range plus or minus a multiple of 293 meters.
Once the code tracking loop is locked, the pseudo-random noise code ca be removed from the satellite signal simply by mixing it with the local oscillator. The de-spread signal then passes to the carrier track loop which demodulates the satellite message by aligning the phase of the channel's local oscillator frequency with the phase of the intermediate or beat frequency. This action is commonly achieved by controlling the frequency of the voltage controlled oscillator. If the phase of the oscillation signal is not correct, a correction signal is applied to the oscillator. The carrier beat phase determines doppler shift between the satellite and receiver indicative of the relative velocity therebetween.
As mentioned above, a GPS receiver derives the pseudo-range from the received phase of the C/A code. More range precision is derivable from the carrier phase and range rate is derivable from the carrier frequency.
The track of each GPS satellite is well known and is published. Further, the information signal transmitted by the satellite describes its exact orbital location. From such orbital location information and the pseudo-range of several satellites, the position of an object on the earth's surface may be unambiguously determined. Three satellites must normally be monitored to obtain two-dimensional position with three-dimensional position being derivable from monitoring of a four satellite set.
Because multiple satellites are necessary to unambiguously determine position in a GPS system, a GPS receiver must monitor more than one satellite signal. In the past, various combinations of multiple channel continuous tracking receivers and switching receivers using one or more hardware channels switched between satellites have been utilized. However, it is generally considered necessary to monitor at least four satellites.
Prior GPS receivers have conventionally used analog processing to determine time of arrival for determining pseudo-range, carrier doppler frequency shift, and to resolve the 50 bit per second DPSK information signal. Only after this information was obtained was it conventionally digitized for computer processing of the information. Recently, there have been systems which have attempted to use digital signal processing techniques in at least part of the information acquisition process in a GPS receiver. However, the digital attempts have uniformly attempted to perform digital processing at a relatively high intermediate frequency requiring custom digital signal processing circuitry operating at sampling frequencies sufficiently high to resolve these relatively high intermediate frequencies. Accordingly, such digital signal processing implementations were uniformly expensive, requiring customized circuitry operating at high frequencies.
The Soviet Union has also implemented a spread spectrum navigation satellite system. The Soviet Union utilizes similar frequencies and data encoding technology to the American GPS system. The Soviet Union GLONASS system utilizes a single 511 bit direct sequence spread spectrum code for all satellites in the system. Each satellite, however, utilizes a unique carrier frequency which identifies the satellite. The GLONASS processing methodology is similar to that necessary for GPS. The RF signal is filtered and down converted and correlated with the matching spread spectrum sequence to collapse the information bandwidth. Because of the similarity of spread spectrum coding and transmitted frequencies, the decoding of GLONASS satellite transmitted signals may be performed in a manner substantially similar to that of GPS satellite signals.
As mentioned above, both existent analog and digital receiving and decoding solutions require relatively complex and expensive hardware operable at relatively high frequencies. There is a need to implement this technology with less expensive solutions to make global positioning receivers more widely available in the commercial market.